Microfibrillated cellulose (MFC), which is a kind of nanofibrillated polysaccharide and which also is known as nanocellulose, is a material typically made from wood cellulose fibers. It can also be made from microbial sources, agricultural fibers etc. In microfibrillated cellulose the individual microfibrils have been partly or totally detached from each other.
Microfibrillated cellulose has a very high water binding capacity and it is thus very difficult to diminish the water content of a slurry comprising microfibrillated cellulose. High water content of a slurry comprising microfibrillated cellulose often prevents usage of MFC in many different application where MFC with high solids would be required.
Today there exist several different methods to remove water from a slurry comprising microfibrillated cellulose. It is for example possible to use different drying techniques. Examples of different drying techniques are evaporation or sublimation, such as spray drying or freeze drying. These techniques are however quite energy demanding and thus not so cost efficient to use in large scale processes. Also, hornification of the microfibrillated cellulose fibers often tends to occur when water is removed with different drying techniques. Hornification is when irreversible bonds between the fibers are formed. When hornification has occurred it is not possible for the fibers to expand and swell in water and the original water bonding capacity of the fibers is thus lost. The hornification may be prevented by addition of chemicals which physically prevent or modify the fibers in such way that the formation of bonds between cellulose fibers are limited or prevented. CA1208631A describes a process to re-disperse dried microfibrillated cellulose by addition of additives that will prevent the fibrils from bonding to each other and thus also prevents hornification of the fibers.
WO2012156880 describes dewatering of microfibrillated cellulose by the use of an electric field. By the use of an electric field it is possible to increase the dewatering of MFC. This is however quite a slow, complicated and energy consuming process.
Previously, mechanical dewatering, such as centrifugation, sedimentation and filtration, in order to remove water from a slurry comprising microfibrillated cellulose, has not been very successfully mainly due to that the characteristics of the slurry. Microfibrillated cellulose at a certain dry content is in form of a gel. One problem with pressing a gel is that the gel will “escape” through all possible openings when the mechanical pressure is applied, i.e. not only water will be removed through the intended openings. Furthermore, if a wire is used during the pressing, the microfibrillated cellulose has a tendency to clog the wire. Filtration is a method that also can be used. However it is very difficult to remove water from a slurry comprising MFC by filtration due to the dense web formed by the slurry which usually requires excessively high pressures and long filtration times.
There is thus a need for an improved process for dewatering of a slurry comprising microfibrillated cellulose.